George MacGinitie & Elkhorn Slough

compiled by
Robert Roy J. van de Hoek
March 21, 2001
Spring Equinox
Malibu, California

The following web page on George MacGinitie is reproduction of his Masters Thesis that was published in American Midland Naturalist in 1935. The web page was created and compiled to guide the curious individual into the realm of "knowledge is power" and "breaking through." The information presented here pertains to Elkhorn Slough but is useful to undertanding the estuaries at Morro Bay, Mugu Lagoon, Carpenteria Marsh, Goleta Slough, Malibu Lagoon, Ballona Wetlands, Ballona Lagoon, Cerritos Wetland, Bolsa Chica Lagoon, Bolsa Chica Wetlands, and many other coastal wetlands with a remnant of California wild nature still found in them. There is something to be said for just plain pure education, knowledge, and curiosity to know about natural landscapes from the perspective of WILD NATURE. That kind of knowledge in all of the writing of George MacGinitie. It also hoped that these web pages will help educate us all about the inter-relationships of the land and the sea, through the eyes of a gifted marine naturalist and historian of marine biology.

The American Midland Naturalist
Volume 16 ....... September, 1935 .......... Number 5

ECOLOGICAL ASPECTS OF A CALIFORNIA
MARINE ESTUARY

Introduction

This paper is an account of general ecological studies made at Elkhorn Slough, a tributary of Monterey Bay, California, on the west coast of North America. The work was carried on over a period of eight and one half years (June 1926 to January, 1935), and was done to obtain a comprehensive view of animal life as a whole, particular attention being given to the relation of one species to another, and the adaptation of each species to its environment.

Most of this investigation was carried on directly in the field .....

Possibly no locality offers a better opportunity for ecological study than does Elkhorn Slough. There is little fresh water disturbance; ....

When one stands on the edges of the bank of the Slough at low tide and looks out across the mud flats, life in the Slough seems quite meager, for about the only living things apparent are the birds.....

The work done in the preparations of this paper was carried on from the ... Kerckhoff Marine Laboratory of the California Institute of Technology.... I am also greatly indebted to my wife, Nettie MacGinitie, for unstinting interest and help both in the laboratory and field.

My thanks are also due the following people for aid as indicated:
...and to Mr. E.F. Ricketts of the Pacific Biological Laboratories at Pacific Grove for help with all groups.

Pollution

Pollution as yet has not become a menace to life in the Slough because the population is so sparse that there is but little sewage. When the Salinas River breaks through into the Slough there is considerable danger of pollution from the sugar factories along the former, and, in addition, the fresh water which afford darker places for the larger crabs, fish, and other animals. However, during the nine years that this investigation has been carried on the inflow of freshwater from the Salinas River has never been serious. ... Man's part in the life chain is interesting only because of its negative influence. Clams are dug by sackfuls. I have seen five sackful of Saxidomus,the finest clam of the Slough and of the Pacific Coast, taken by four Italians, and nine sacksful taken by Japanese. Japanese comb out everything which is not protected by law, and the law protects only crabs and one clam - and that not an important one. In digging for clams all other life is destroyed wholesale. When man is included in the food chain, the prospects of life continuing in its present abundance is not bright.

Nocturnal Activities

In certain ways, however, the Slough is different at night. Pachygrapsusleaves his shallow burrow along the bank and Hemigrapsuscomes up from his daytime hiding place. Both of these crabs are especially active at night and are much in evidence. Pachygrapsusstays along the shores, but Hemigrapsus usually perches on the Zostera,drawing several blades together and buoying itself up. So far as could be determined these crabs were not feeding directly on the Zostera, but were picking at things here and there, searching for whatever suited their gastronomic inclinations. Pugettia seem relativeley scarce during the day, but at night they are everywhere and busily feeding. They, too, buoy themselves up with the Zostera, but stay lower down than Hemigrapsus does. The Cancer crabs stay on the bottom, as during the day, but they seem to be much more active at night.

Fish, and the surface living crustacea, show the greates increased activity of all the animals. Striped Bass may be heard splashing about. Smelt, cabezons, and flounders are everywhere about one's legs or under foot. When the direct beam of a flashlight is once fixed on a fish it remains perfectly still and is easily netted. Several smelt, cabezons, and gobies were netted in this way.

No birds were about and, with the exception of an occasional squawk from a Great Blue Heron which was roosting in a nearby tree, none were heard. Night Herons feed at night, but are relatively silent about it. The absence of the fish-eating birds may in part account for the greater activity of fishes at night.

Phyletic Catalog of Animals, With Natural History Notes

The greatest obstruction to the completion of this paper has been a taxonomic one. In some cases I have been waiting eight years to have species named by those who are authorities on certain groups....In other words, before this investigation, forty-five, or over 21%, of the animals found at Elkhorn Slough were new species or represented an extension of range. I can not refrain from remarking here that in view of this condition it seems that some attention should be paid to our own ocean fronts, which could be done at much less expense than can the naming of a few new animals found on expeditions to the Galapagos, Bermudas, or the South Seas.

ARTHROPODA
Class CRUSTACEA

This is one of the best represented groups at the Slough, and also one of the most interesting... The Cancer crabs are taken in considerable numbers by the campers... The proprieter of the camp ground on the north side of the highway bridge states that he measured one which was 13 inches across the carapace. Some of the Cancer crabs seems to be more plentiful at certain times of the year, at least they are more in evidence, and I believe that they migrate in and out of the Slough.

The shore crabs are plentiful along the banks; Pachygrapsus crassipes may be obtained in large numbers. Hemigrapsus oregonensismay be obtained in almost any quantity from the ditch leading back from the large pump at the salt works. Hemigrapsus nudusis most plentiful along the shore and among the rocks at Station 4, but it is not nearly so plentiful as the two preceding species.

Family Grapsidae
Genus Pachygrapsus
Pachygrapsus crassipesRandall, 1839
This shore crab is very numerous in burrows along the entire east bank, and among the rocks, and around the pilings and rocks at Station 7.

Many individuals of this species are covered with barnacles, bryozoa, etc. One had so many barnacles that it could not move one of its eyestalks and one leg. The other eyestalk, as well as one chela and the first leg, was missing, and a small Mytilus edulis was growing near the missing eyestalk and another at the base of the missing leg.

These crabs may be seen at the mouths of their burrows along the bank, but they usually vanish if one approaches near them. Sometimes they perch outside their burrows and "blow bubbles," the continued respiratory movements causing the entire forward portions of the creatures to be surrounded by foam. Speciemens may be obtained by locating a burrow along the bank, then pushing a shovel blade down back of the entrance, and then, as the crab runs out of the burrow when the shovel handle is lowered it may be caught with a net.

Pachygrapsus crassipesis a scavenger as an adult, but juveniles pick algal growth by means of their chelae and eat it. One adult was found holding a living young Cancer anthonyiand eating it piecemeal. The mouth parts had been partly eaten and the shore crab was very reluctant to release its victim...

Genus Hemigrapsus
Hemigrapsus nudusDana, 1851
This species is common among the rocks at Stations 4 and 7. Unlike the other shore crabs, its range at the Slough is extremely limited, and, unlike the above species, it does not inhabit burrows along the bank.

Genus Hemigrapsus
Hemigrapsus oregonensis(Dana), 1851 Rathbun 1904
This shore crab is the most plentiful of all the crabs at the Slough and is found at all stations. Although it prefers beds of Zosteraor algae, such as EnteromorphaorPolysiphonia,as places of refuge, many individuals are found on the open bottom. Smal specimens often scuttle into the burrows of Callianassa, Upogebia and Urechis for temporary places of refuge. Great numbers collect in the ditch above the highway bridge where the pump of the salt works lifts water from the Slough and empties it into a ditch leading to the evaporating works.

When the tide is out Hemigrapsushas a curious habit of collecting in considerable numbers on small exposed areas, such as mounds of Enteromorpha or of mud left by clam diggers, and exposing themselves to the air.

This crab feeds mostly at night, and by the aid of a flashlight one may see individuals buoyed up on several blades of Zostera which they hold together with the legs. As they move about they reach out and draw in new blades before releasing others. One crab was found tightly grasping a hermit crab, which it probably devoured.

MOLLUSCA

Because of the many different conditions of bottom soil represented at the Slough the molluscs are plentiful both in species and individuals, and they offer one of the best examples of adaptive deployment to be found. Some adhere strictly to one kind of bottom; whereas others apparently try to adjust themselves to all.

Genus Saxidomus
Saxidomus nuttalliiConrad 1837
This large clam is found in limited numbers at Stations 2,3,4,5, and 6. Because of its value as a food clam it is being rapidly exterminated by the Italians and Japanese who have taken it by the sacksful. This is one of the few larger clams at the Slough which has neither parasite nor commesal.

Genus Schizothaerus
Schizothaerus nuttalliiConrad 1837
Of the large clams at the Slough this is the most plentiful one...Schizothaerusis an edible clam, but the gaping of the valves makes it unsuited for shipping or market value. At the Slough this clam is always infected with the larvae of a tetraphyllid cestode, the primary host of which is the Sting Ray, Myliobatis californcus. One hundred forty cysts of this cestode were taken from one clam, and each cyst may contain from one to five larvae. A law intending to restrict the taking of Saxidomus nuttallii was passed by the State legislature of California, but the technical name Schizothaerus nuttallii was used, so this tough-necked, undesirable clam was protected and Saxidomus nuttallii is being exterminated.

At the Slough Schizothaerus nuttalli has spawned in April, and well-filled gonads have been noted during the first of June.

Family Aglajidae
Genus Navanax
Navanax inermisCooper 1862
In July, 1927, one specimen of this tectibranch was found on the sand at Station 2. Prior to this (MacGinitie, 1930b) it had not been reported from north of San Diego or Santa Catalina. In July, 1928, two more specimens were found at Stations 2 and 3. Two more specimens were found at Station 3 in April, 1930, and a single specimen 6 inches in length was found at this same station in July.

Although only six Navanaxwere found, the fact that they were found over a period of three years indicates that their occurrence at Elkhorn Slough is not a matter of chance.

At the Slough Navanaxeggs were found in July at Station 7. Two Navanaxtaken to the laboratory in July mated shortly after being placed in an aquarium, and during the night laid two large skeins of eggs. At Newport Bay and Anaheim Slough, in Southern California, eggs of Navanaxmay be found during the entire year. The eggs, which are white, are laid in a long coiled string embedded in a skein of transparent jelly which is about 4 inches long.

Family Aplysidae
Genus Tethys
Tethys californicusCooper 1863
On the coast of California there are two species of sea hares, whose ranges may be separated roughly by Point Conception. The descriptions of these animals are very poor, for mulluscan taxonomists on the whole seem to be afraid to take into consideration anatomical structure rather than shell shape, and Tethys has only a vestigial shell, which is internal. Actually, the description of Tethys californicusfits the southern form better, and the Tethys ritterifits neither. However, since a discussion and description of the two animals is out of place here, the northern sea hare, or the one found in Elkhorn Slough, will be called Tethys californicus,as this is the name applied to it by the marine personnel of the Hopkins Marine Station.

The northern species reaches a weight of 15 pounds, and one, while crawling in an aquarium, measured 480mm. long, 260mm. high, and 170mm. broad.

This species may occur very abundantly at the Slough at certain times, and other times be conspicuous by its absence. In the summer of 1926 none at all were seen, in spite of special effort to find them, but during the summer of 1927 fifty nine specimens were counted. Specimens were found in January, February, April, July, August, and November. On the night of November 28, 1930, when there was an exceptionally low tide, hundreds of sea hares, including many small-sized ones, were found. Dozens of them had been left stranded by the low tide. Some were mating, and both fresh and old eggs were found.

Although on the outside beach Tethyseats seaweed, while in the Slough it feeds on Zostera. It consumes vegetation in great quantities. This is rather surprising for a marine animal, for most of them feed sparingly, at least as compared to land animals. The indications are that the sea hares live to be two years old, spawning the second year.

The mass of eggs mentioned above was kept at the laboratory, and although conditions were somewhat adverse, all the eggs hatched. The larvae hatch as veligers, and the period of mortality is undoubtedly at the time they metamorphose and settle to the bottom, for apparently thes tectibranch are not fed upon by other animals after they are once well started.

In contrast to the above statements in regard to Tethys californicus at Elkhorn Slough it may be noted that the southern form laid 478 million eggs in four months and one week, in a total of 27 layings (MacGinitie, 1934b). I am quite sure that the northern form does not lay more than twice, and I think, in general, only once. The southern form, at least, dies afater the completion of egg-laying the second year, and I believe that teh northern form does likwise, although I am not sure of this latter point.

The potential space required for the third or fourth generation of offspring of either the northern or southern form (the southern not exceeding 6 pounds in weight) may furnish amusement and recreation for those who care to make such computations.

VERTEBRATA
Class Aves

Tide flats are always a favorite feeding ground for shore birds and sea birds; therefore, the area dealt with in this paper has its quota of birds which are representative of such localities. The daily feeding of the great number of birds which one sees at the Slough (and which are listed below), can not fail to have a considerable effect on the animal life there and I believe that this paper would be incomplete with this list of birds and something of their feeding habits.

Of all the birds the gulls, which are represented mainly by the Western Gull and the Ring-billed Gull, are the most numerous. In the summer time the gulls nest on rocks off shore or farther north along the coast and inland. While nesting their food consists mainly of eggs of sea birds, which the gulls obtain by robbing other nests, and of fish which they obtain by robbing other sea birds as the latter bring food to their nestlings. But there are always a considerable number of gulls which seem to be permanent residents at the Slough during the summer, no doubt because, for some reason, they have not mated. After the nesting season the gulls appear in thousands, and often in the fall of the year one of the tide flats may be covered with these birds. Within the reaches of the Slough the gulls fed upon anything they can obtain in the way of flesh, living or dead. They scour the tide flats at low tide, picking up any stranded life and taking considerable toll of the young clams which have the habit of coming to the surface when exposed by the tide.

Next in importance, from the point of view of their demand upon the life of the Slough for food, are the California Brown Pelicans. As far as can be determined, these birds live entirely upon fish, which they take from the water by diving for them. Most of their activities are carried on at the mouth of the Slough,....fishing or resting upon the spits.

The Great Blue Heron follows the Pelican in importance as a feeder on animal life at the Slough. A considerable number of these great birds nest in the Eucalyptus grove on the south side of the Slough just above the higway bridge. Except during the nesting season, when they may be seen feeding at any time, these birds feed mainly early in the morning or late in the evening.

Folowing the Great Blue Heron in importance come Least Tern, several pairs of which nest on the dunes west of Station 2. These birds, although small, are tireless fishers, and from six to a dozen may be seen at any time flying about near the surface in search of fish. When they sight a fish they usually flutter above it for a moment and then fall like a plummet, sometimes going completely out of sight under the water. They become quite tame and often drop into the water within a few feet of a boat or a person. On one occasion a young tern which was learning to fly alighted on a tide flat with a few feet of me, and the parent birds were observed as they brought food to the young bird and fed it.

In addition to the birds discussed above, Scoters, Loons, and Cormorants fish to some extent in the Slough. The Scoters are important because they feed on small crabs as well as fish.

The shore birds are very numerous in the latter part of August until the following spring. Their food consists mainly of worms and small crustacea. Thus, judging by their numbers, they are of considerable importance in the food chain......


1